In the highly competitive container industry, a container must be made with the absolute minimum of material, which means that the finished container must have extremely thin walls. At the same time, however, a container must have sufficient strength to avoid collapsing or losing its cylindricity and, hence, its structural integrity.
Metal containers are generally made as two-piece containers. In the two-piece form, the container body and one end are formed integrally and a separate end is later attached to the open end of the container. Two-piece containers may be made in several ways. One of these ways includes the wall-iron process wherein a shell cup is produced from sheet metal by conventional drawing techniques and is then redrawn to a cup of longer length and smaller diameter. The redrawn cup is then wall-ironed to produce the required body length and wall thickness. Containers manufactured by this process are commonly referred to as drawn-and-ironed ("D&I") containers.
A second way of forming a container body with an integral end is the impact-and-ironed forming process wherein a short cup of proper diameter but with a heavy wall thickness is formed by the impact-extrusion-forming process. The impact-extruded short cup is then pushed through ironing dies to reduce the wall thickness and increase the length of the cup to the required length for trimming and flanging operations.
Where a container body is formed by the drawn-and-ironed process, or by the impact-and-ironed forming process, a cup, formed as described above, is reformed to a container body of longer length by an ironing procedure whereby the cup is moved through a series of ironing rings so as to reform the cup into intimate contact with the punch and to iron the cup sidewall to the proper container body length. Conventional methods often incorporate redrawing and ironing into one machine.
After the oversized cup on the ironing punch has been ironed by passing it through the series of ironing rings, the container body so formed must be removed or "stripped" from the ironing punch. To remove the container from the ironing punch, the stripping operation usually employs compressed air directed through one or more passageways which are integrally designed into the machinery, as the primary means of stripping the container bodies from the punch. Mechanical strippers are sometimes employed to ensure that the container bodies are actually removed from the punch.
In the stripping of the container from the ironing punch, a circumferential outward bulge is commonly formed adjacent the lower portion or closed end of the container. This bulge, commonly referred to as the "stripper bulge," has been found to decrease the axial strength of the container, thereby increasing its propensity to collapse or lose its cylindrical integrity. Stripper bulges are found on substantially all containers manufactured by the methods including the wall-ironing process, regardless of the manufacturer. Contour tracings of numerous containers from several manufacturers reveal that the stripper bulge is from 0.004 to 0.010 inches greater in diameter than the straight cylindrical sidewall of the containers.
The majority of conventionally formed containers are sold to industry in an "as made" state. Heretofore, no attempts have been made to improve the integrity of ironed containers by reworking the lower portion of the container to reduce or eliminate the stripper bulge.